In mechanical structures, parts are often connected together with the help of couplings. In general, a coupling is defined as being a mechanical part that serves to join together two other parts, or to reinforce a part. In structures where weight saving is sought after, for example in a turbomachine, couplings are often made of composite materials having a polymer matrix reinforced by fibers such as carbon fibers, glass fibers, or Kevlar® fibers. Such composite materials present density that is considerably smaller than that of the light alloys commonly in use, and they present better mechanical performance (in terms of stiffness and resistance to rupture).
By way of example, a coupling may be annular in shape. Such an annular coupling may be of the type used for joining a retention casing with an air inlet sleeve or an intermediate casing shroud. It may possess at one of its axial ends (i.e. at one end along its axis of symmetry) an angled region or “flange” joining the tubular central portion of the coupling at the end of the coupling, which flange extends substantially radially. This end includes holes for receiving bolts enabling the annular coupling to be secured with the structure against which said end comes into contact.
More-recent composite materials are made by preparing a three-dimensional fiber preform, i.e. by weaving or braiding fibers in three dimensions. Such a preform is subsequently densified with a polymer in order to make the finished composite part, in which the yarns of the preform are embedded within a solid polymer matrix.
By way of example, one known technique for achieving such densification is liquid impregnation: a distinction is drawn between infusion and injection. With the infusion technique, the preform is placed between a mold half and a cover, a chemical precursor of the polymer is then infiltrated while in liquid form via one end of the preform while a vacuum is established between the mold half and the cover. Under the action of the vacuum, the precursor diffuses throughout the preform, after which it is polymerized by heat treatment so as to be solidified. In the injection technique, the preform is placed in a mold and then the liquid precursor is injected into the mold via a plurality of points until the entire mold is filled (the resin transfer molding (RTM) method), and is then polymerized by heat treatment.
Another known technique for densifying the preform is chemical vapor infiltration (CVI). The preform is then placed in an enclosure and a gaseous phase containing a precursor of the polymer is admitted into the enclosure. Under the temperature and pressure conditions that are established inside the enclosure, the gas diffuses into the preform where it transforms into a polymer on coming into contact with the fibers of the preform.
In order to ensure that the coupling has adequate mechanical behavior, it is necessary for the outer surfaces thereof to match exactly the surfaces of the part with which the coupling comes into contact and to which it is assembled.
Unfortunately, the geometrical specifications for coupling assemblies (or coupling reinforcements) are very varied. The technique consisting in making a different mold for each assembly configuration is not economically viable. Furthermore, for a coupling that possesses a flange, in particular an annular coupling in which the flange is circumferential, the preform can never be deformed sufficiently to match exactly the outlines of the mold in the corners of the flange, since the radius of curvature in these corners is too small. As a result, after densification, the corners of the coupling are zones that are rich in polymer, and therefore mechanically weaker.
In order to accommodate all existing geometrical specifications for couplings, it is therefore necessary to machine those regions of such couplings that come into contact with the surfaces of adjacent parts.
Unfortunately, it is not acceptable to machine the coupling since any such machining would cut through the fibers of the preform, thereby compromising the mechanical integrity of the coupling, since it is the fibers that provide it with mechanical strength.